1. Field of the Invention
The present invention relates to a light emitting display and a Thin Film Transistor (TFT), and more particularly, to a TFT having improved driving ability by reducing off-current and a light emitting display having enhanced image quality using the TFT.
2. Description of the Related Art
Generally, an active matrix type light emitting display includes at least a single data line for transmitting data signal, at least a single scanning line for transmitting a selecting signal, and a plurality of image pixels electrically connected to the data line and the scanning line. Generally, each image pixel includes a light emitting device, a capacitor for storing voltage corresponding to the data signal transmitted from the data line, and at least two Thin Film Transistors (TFTs) for controlling the light emitting device. As described above, the structure of the image pixel including two TFTs and a single capacitor is ‘2TR+1Cap’ structure.
In the 2TR+1Cap structure, one of the two TFTs serves to as a switching device for responding the selecting signal and transmitting the data signal to the light emitting device, and the other serves to as a driving device for supplying electric current to corresponding to the selected data signal to the light emitting device. The TFTs are manufactured to have a semiconductor layer, a gate insulating layer, a gate electrode, and source/drain electrodes laminated in turn, and are generally divided into a staggered type TFT and a coplanar type TFT. Moreover, according to the formation position of the gate electrode with respect to the source/drain electrodes, the TFTs are divided into an upper gate structured TFT and a lower gate structured TFT.
For example, the upper gate structured TFT includes a semiconductor layer formed on a substrate, a gate insulating layer formed on the semiconductor layer, a gate electrode formed on the gate insulating layer, and source/drain electrodes. The semiconductor layer includes source and drain regions doped with impurities of a predetermined concentration, and a channel region formed between the source and drain regions. The gate electrode is generally formed on the gate insulating layer on the channel region, and the source/drain electrodes are respectively connected to the source and drain regions through contact holes while interposing interlayer-insulating layer on the gate electrode.
In the light emitting display including the image pixels having the two TFTs and the capacitor, when leakage current of the switching TFT turning on/off a driving TFT is large, the capacitor cannot maintain data voltage uniform. Thus, in the light emitting display, crosstalk, flicker, and so on are caused, and further image quality of the light emitting display may be deteriorated.
In order to overcome the above-mentioned problems and improve the image quality of the light emitting display including the image pixels having the 2TR+1Cap structure, addition of complementing circuits to the image pixels is proposed.
However, since, when the complementation circuits are added to the image pixel structure, a plurality of TFTs must be added, driving method thereof becomes relatively complex in comparison to the conventional image pixel structure. Moreover, since, in the image pixel structure including the complementation circuit, there is a high possibility of leakage current through the path from the capacitor to the plural TFTs, the ability of the capacitor for maintaining voltage is relatively deteriorated. In other words, although the complementation circuit is added to the image pixel structure, the image quality of the light emitting display may be deteriorated.
Additionally, terminals of every device constituting the image pixel are affected by kickback voltage generated by the peak-to-peak of a gate signal used in a single image pixel, that is, the voltage difference raging from EL VDD to EL VSS. Moreover, when the kickback voltage, generated by a terminal to which the capacitor is connected, is increased, voltage for driving the light emitting devices is affected so that the image quality is deteriorated.